LiNbO3 and LiTaO3 Coating Effects on the Interface of the LiCoO2 Cathode: A DFT Study of Li-Ion Transport

In solid-state batteries, the interface between cathodes and solid electrolytes is crucial and coating layers play a vital role. LiNbO3 has been known as a promising coating material, whereas recent studies showed its degradation via releasing oxygen and lithium during cycling. This computational study addresses the elucidation of essential characteristics of the coating materials by examining LiNbO3 and its counterpart LiTaO3 interfaces to a representative layered cathode, LiCoO2. Employing the interface CALYPSO method, we constructed explicit models of both coatings on LiCoO2. Our findings indicate that LiTaO3 offers easier Li+ migration at the interface due to the smaller difference in Li adiabatic potential at the interface, whereas LiNbO3 more effectively suppresses oxygen activity at high delithiation states via lowering the O 2p states. This comparative analysis provides essential insights into optimizing coating materials for improved battery performance.

Table S2.Comparison between surfaces selected from X-cut, Y-cut and Z-cut.

Diffusion properties near the interface in LCO and coating materials
Beyond the interface itself, the diffusion properties near the interface are also crucial to the electrochemical performance. 5We therefore compared the Li + diffusion properties between the LCO slab and coating layers.For a clear comparison with the diffusion properties in the bulk region, Li + diffusion properties in bulk LCO, LiNbO and LiTaO are calculated and compared with previous work. 4In terms of Li + diffusion in LCO, two types of diffusion pathways are considered:

Figure S1 . 3 .Figure S2 .
Figure S1.Schematic representations of interface models for the LiNbO coating on LCO (a) and LiTaO coating on LCO (b).Charge density difference calculated for (c) LiNbO interf and (d) LiTaO interf.Electronic charge depletion and accumulation regions are represented by cyan and yellow, respectively.The isosurface value was set to 0.01 eV Å -3 .

Figure S3 .
Figure S3.Band gap (Eg) as a function of strain in LiNbO and LiTaO bulk.Strain types as follows: Uniaxial strains along a axis, Uniaxial strains along c axis, biaxial strains along a and b axes, biaxial strains along a and c axes, and isotropic strains.

Figure S4 .
Figure S4.PDOS of surface O and Co near the highest-occupied states in (a) LiNbO interf and (b) LiTaO interf.
dumbbell hopping (ODH) and tetrahedral site hopping (TSH).The ODH pathway (FigureS6(a)) represents Li + migration via single vacancy, whereas TSH (FigureS6 (b)) shows Li + migration via a tetrahedral site when there are multiple Li vacancies available.6Ea for ODH and TSH are 0.69 eV and 0.23 eV, respectively, in good agreement with previous calculations.[6][7][8]Li migration in LiNbO and LiTaO was then analyzed, considering only the nearest diffusion pathways (green images in FigureS6 (c)).Notably, the Ea for LiNbO and LiTaO measured in this work are 1.06 eV and 1.02 eV, respectively, in good agreement with previous research.4FiguresS5 (a) and (b) demonstrate the variation in Ea within the coating layers, ranging from approximately 0.3 eV to 1.3 eV.This spread is attributed to local strain effects, which can either enhance or impede Li + diffusion due to changes in the local lattice space upon coating contact.For instance, within the LiNbO coating, a significant energy barrier of 1.49 eV is noted, alongside an extended Li + diffusion distance which increases from approximately 3.9 Å (FigureS8 (a)) in bulk LiNbO to about 4.5 Å.Conversely, a substantially lower Ea of ~0.32 eV is observed, with a markedly shortened Li + diffusion distance, decreasing from ~3.9 Å to ~2.7 Å.These findings are consistent with our supplementary NEB calculations, wherein we applied tensile and compressive strain to alter the Li + diffusion distance in bulk LiNbO, as depicted in FigureS7.Additionally, certain lengthy diffusion pathways with low Ea were identified, as illustrated in FigureS5 (a) and (b).We analyzed the Li + diffusion pathway in bulk LiNbO and found a tendency for Li + to bond with O (Li labeled '3' and O indicated by a cross in Figure S8 (a)), with the breaking of this bond leading to the highest energy barrier, as seen in Figure S8 (b).It is posited that longer distances between Li and this specific O (originally 0.96 Å from Figure S8 (a)) correlate with weaker bonding interaction, consequently leading to a lower Ea value.Upon examining pathways with extended diffusion distances of yet lower Ea, it was discovered that this atomic separation increased to over 1.5 Å, elucidating the observed lower Ea values.Regarding Li + diffusion on the LCO side, FiguresS7 (c) and (d) illustrate that the Ea along the ODH pathway decreases from the middle of the slab (reflecting bulk properties) toward the interface.For example, in LiNbO interf, Ea decreases from 0.63 eV to 0.47 eV.Interestingly, Li + diffusion along the TSH pathway does not change much, maintaining at around 0.17 eV.Such negligible variation along the TSH pathway under the strain effect has been observed in previous DFT calculations of Li + diffusion.9Consequently, the presence of the coating is expected to facilitate Li + diffusion in LCO due to the lower Ea along the ODH pathway and within the coating layer itself.

Figure S5 .
Figure S5.Li + diffusion pathway (a) in LiTaO coating layers, (b) LiNbO coating layers; along the ODH pathway in LCO in (c) LiNbO interf and (d) LiTaO interf; along TSH in LCO in (e) LiNbO interf and (f) LiNbO interf.Yellow, red and blue circles show the diffusion images of Li + .Red crosses mark the additional Li vacancy for the TSH diffusion pathway.All plots share the same color code as Figure 1.

Figure S6 .
Figure S6.Diffusion pathway for (a) ODH and (b) TSH in LCO.(c) Green circles show the first nearest neighbor (NN).

Figure S7 .
Figure S7.Migration energy barrier (Ea) as a function of strain in LiNbO and LiTaO bulk.Strain types include uniaxial strains along a axis, uniaxial strains along c axis, biaxial strains along a and b axes, biaxial strains along a and c axes, and isotropic strains.

Figure S8 .
Figure S8.(a) Migration pathway of Li + in LiNbO bulk.Diffusion distance is indicated in green.Li + and O bond distance is indicated in red.Numbers indicate the reaction coordination in (b).A cross marks the closed oxygen atom with the diffusing Li + .